Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to light of a certain wavelength. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be imaged onto the photoresist. After exposure the photoresist is developed to produce an image that corresponds to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system that illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example. The apparatus further includes a mask stage for aligning the mask, a projection objective (sometimes also referred to as ‘the lens’) that images the illuminated field on the mask onto the photoresist, and a wafer alignment stage for aligning the wafer coated with the photoresist.
A desire in the development of projection exposure apparatus is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to a high integration density, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus.
Various approaches have been pursued in the past to achieve this aim. One approach has been to reduce the wavelength of the projection light used to image the circuit pattern onto the photoresist. This exploits that fact that the minimum size of the features that can be lithographically defined is approximately proportional to the wavelength of the projection light. Therefore the manufacturers of such apparatus strive to use projection light having shorter and shorter wavelengths. The shortest wavelengths currently used are 248 nm, 193 nm and 157 nm and thus lie in the deep (DUV) or vacuum (VUV) ultraviolet spectral range. The next generation of commercially available apparatus will use projection light having an even shorter wavelength of about 13.5 nm which is in the extreme ultraviolet (EUV) spectral range. An EUV apparatus contains mirrors instead of lenses because the latter absorb nearly all EUV light.
Another approach is to improve the illumination of the mask. Ideally, the illumination system of a projection exposure apparatus illuminates each point of the field illuminated on the mask with projection light having a well defined spatial and angular irradiance distribution. The term angular irradiance distribution describes how the total light energy of a light bundle, which converges towards a particular point on the mask, is distributed among the various directions of the rays that constitute the light bundle.
The angular irradiance distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be imaged onto the photoresist. For example, relatively large sized features may involve a different angular irradiance distribution than small sized features. The most commonly used angular irradiance distributions are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a pupil surface of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil surface. Thus there is only a small range of angles present in the angular irradiance distribution of the projection light, and all light rays impinge obliquely with similar angles onto the mask.
Different approaches are known in the art to modify the angular irradiance distribution of the projection light in the mask plane so as to achieve the desired illumination setting. For achieving maximum flexibility in producing different angular irradiance distribution in the mask plane, it has been proposed to use mirror arrays that determine the irradiance distribution in the pupil surface.
In EP 1 262 836 A1 the mirror array is realized as a micro-electromechanical system (MEMS) including more than 1000 microscopic mirrors. Each of the mirrors can be tilted about two orthogonal tilt axes. Thus radiation incident on such a mirror device can be reflected into almost any desired direction of a hemisphere. A condenser lens arranged between the mirror array and a pupil surface translates the reflection angles produced by the mirrors into locations in the pupil surface. This illumination system makes it possible to illuminate the pupil surface with a plurality of spots, wherein each spot is associated with one particular mirror and is freely movable across the pupil surface by tilting this mirror.
Similar illumination systems using mirror arrays are known from US 2006/0087634 A1, U.S. Pat. No. 7,061,582 B2 and WO 2005/026843 A2.
Although illumination systems using mirror arrays are very flexible for modifying the angular irradiance distribution, the uniformity of the spatial and angular irradiance distribution over the illuminated field in the mask plane is still an issue. Future generations of illumination systems are likely to involve a very low field dependency of these quantities.
Some of the approaches that have been developed to reduce the field dependency focus on the optical integrator that is usually used in illumination systems to produce a plurality of secondary light sources. The light beams emitted by the secondary light sources are superimposed by a condenser onto a mask plane or onto a field stop plane which is optically conjugate to the mask plane. The optical integrator usually includes one or more arrays of optical raster elements producing the light beams that are associated with the secondary light sources. One or more optical raster elements which are exclusively associated with such a light beam form an optical channel that is independent from other optical channels. Since each light beam associated with an optical channel completely illuminates the mask or field stop plane, optical elements located within the optical channels can be used to modify the illumination properties.
For example, U.S. Pat. No. 5,615,047 describes a plate which is arranged in front of an optical integrator and includes a plurality of filter areas each being associated with a single optical channel of the optical integrator. Since the position of the filter element is optically conjugate to the mask or field stop plane, the transmissivity distribution of a filter area can be selected such that a uniform spatial irradiance distribution at the mask or field stop plane is obtained.
Also U.S. Pat. No. 6,049,374 proposes to use absorptive filter elements that are associated with particular channels of the optical integrator.
US 2009/0021715 A1, which is assigned to the applicant of the present application, describes an illumination system in which undesired residual field dependencies of the angular irradiance distribution are removed. To this end optical elements such as prisms placed in individual optical channels change certain optical properties of the light beams associated with these optical channels.
However, there is still a desire for improvements of illumination systems in particular with regard to the field dependency of the angular irradiance distribution of the projection light impinging on the mask.